Advances in polymer chemistry and technology over the last few decades have enabled the development of high-performance polymeric fibers. For example, liquid-crystalline polymer solutions of rigid-rod and semi-rigid-rod polymers can be formed into high strength fibers by spinning liquid-crystalline polymer solutions into dope filaments, removing solvent from the dope filaments, washing and drying the fibers; and if desired, further heat treating the dried fibers. One example of high-performance polymeric fibers is para-aramid fiber such as poly(paraphenylene terephthalamide) (“PPD-T” or “PPTA”).
Fiber strength is typically correlated to one or more polymer parameters, including composition, molecular weight, intermolecular interactions, backbone, residual solvent or water, macromolecular orientation, and process history. For example, fiber strength typically increases with polymer length (i.e., molecular weight), polymer orientation, and the presence of strong attractive intermolecular interactions. As high molecular weight rigid-rod polymers are useful for forming polymer solutions (“dopes”) from which fibers can be spun, increasing molecular weight typically results in increased fiber strength.
Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole, para-phenylenediamine and terephthaloyl dichloride are known in the art. Hydrochloric acid is produced as a by-product of the polymerization reaction. The majority of the fibers made from such copolymers have generally been spun directly from the polymerization solution without further treatment. Such copolymers are the basis for a high strength fibers manufactured in Russia, for example, under the trade names Armos® and Rusar®. See, Russian Patent Application No. 2,045,586. However, the copolymer can be isolated from the polymerization solvent and then redissolved in another solvent, typically sulfuric acid, to spin fibers.
Previously, it was not appreciated that fibers derived from copolymers of 5(6)-amino-2-(p-aminophenyl)benzimidazole, para-phenylenediamine and terephthaloyl dichloride are prone to thermal damage when drying in a continuous process even at low drying temperatures. This thermal damage can create yarns having depressed tensile properties. Therefore, what is needed are new methods to dry and further heat treat these copolymer fibers without damage.
Known processes for making copolymer fibers directly from polymerization solution, while producing a good product for use in ballistic and other aramid end-uses, are very expensive with very poor investment economics. As such, there is a need in the art for manufacturing process wherein the copolymer is solutioned in a common solvent, such as sulfuric acid which has both improved economics compared to processes known in the art and provides copolymer fibers having superior long-term physical properties.